Piezoelectric materials are well know for their ability to generate an electrical output when they are placed under mechanical stress. The frequency and magnitude of an electric signal developed across such a piezoelectric material are directly proportional to the frequency and magnitude of a force applied to the material. There are characteristics of the piezoelectric material's mechanical system that are analogous to electrical systems. For example, the material's response to a mechanical force attempting to bend it is related to the mechanical force in much the same way an impedance is related to the electrical potential across the impedance. Piezoelectric materials can be characterized by their inductive, capacitive and resistive properties. There are numerous applications of piezoelectric materials' ability to generate electrical signals which vary with applied force. When the applied force is not changing, there is no output from the piezoelectric material.
Piezoelectric materials also undergo mechanical motion when electrical potentials are applied across them. There are several applications that make use of this characteristic for the purpose of changing electrical energy into mechanical energy. A piezoelectric material's ability to react to electrical energy can be used to detect the amount of an applied mechanical force. A piezoelectric device is coupled to a function generator capable of generating, for example, a sine wave of a desired frequency and amplitude. If no external force is applied to the piezoelectric material, the material will deflect back and forth at the same frequency and relative magnitude to the applied signal. If an external mechanical force is applied to the material when the electrical signal is applied, the mechanical force will tend to inhibit the electrically induced motion of the piezoelectric material. This resistance to motion will be reflected back to the electrical system as an impedance to current flow and the applied electrical energy not manifested as motion is dissipated as heat in the piezoelectric material. The source subsystem can be constructed in such a way as to detect this extra impedance, for example, by monitoring changes in current flow, or, in a constant current system, by changes in the voltage across the piezoelectric element. Either way, the force applied to the piezoelectric material is reflected back to the source of the applied electrical potential.
According to one aspect of the invention, a monitoring method comprises providing a first, piezoelectric device which is subject to a force to be monitored, determining the first device's output, and correlating the first device's output with the force.
Illustratively according to this aspect of the invention, determining the first device's output comprises providing electrical signals for exciting the first device and correlating the first device's output with the force comprises determining the first device's response to the electrical signals .
Illustratively according to this aspect of the invention, determining the first device's output and correlating the first device's output with the force comprise sweeping the signal frequency and determining the first device's impedance in response to the swept signal frequency.
Further illustratively according to this aspect of the invention, determining the first device's output comprises providing a switch having a first state corresponding to a dynamic mode of operation of the first device when the force applied to the first device is being substantially modulated and a second state corresponding to a static mode of operation when the force applied to the first device is not being substantially modulated.
Illustratively according to this aspect of the invention, switching of the switch to the second state causes a signal to be applied to the first device and causes the impedance of the first device to the applied signal to be determined.
Further illustratively according to this aspect of the invention, determining the first device's output and correlating the first device's output with the force comprise providing electrical signals for exciting the first device and determining the impedance of the first device to the electrical signals.
Additionally illustratively according to this aspect of the invention, providing electrical signals for exciting the first device and determining the impedance of the first device to the electrical signals comprise sweeping the signal frequency and determining the first device's impedance in response to the swept signal frequency.
Illustratively according to this aspect of the invention, determining the first device's output and correlating the first device's output with the force comprise determining from the impedance the force on the first device.
Illustratively according to this aspect of the invention, the method further comprises determining from the determined force whether an object is on the first device.
Additionally illustratively according to this aspect of the invention, providing a first device comprises providing a plurality of said first devices coupled together in an array.
Illustratively according to this aspect of the invention, providing a first, piezoelectric device comprises providing a flexible piezoelectric film.
Further illustratively according to this aspect of the invention, providing a flexible piezoelectric film comprises providing an array of said first devices.
Illustratively according to this aspect of the invention, providing a first device comprises providing a coaxial cable including a center conductor, a shield, and between the center conductor and shield, a piezoelectric material.
Further illustratively according to this aspect of the invention, providing a first device comprises providing a piezoelectric ceramic transducer.
Additionally illustratively according to this aspect of the invention, providing a plurality of said first devices coupled together in an array further comprises providing a third device for holding the plurality of first devices in the array on a detection surface.
Illustratively according to this aspect of the invention, providing a piezoelectric transducer comprises custom fabricating a piezoelectric transducer to a particular size, shape, and electrical properties.
Illustratively according to this aspect of the invention, the method further comprises orienting the first device at a strategic load-bearing point.
Further illustratively according to this aspect of the invention, providing a first device comprises providing a plurality of first devices coupled together and oriented at a plurality of strategic load bearing points so as to permit an object's location to be determined more accurately.
Additionally illustratively according to this aspect of the invention, determining the first device's output and correlating the first device's output with the force comprise detecting a relatively large signal generated by the first device, then determining if the signal generated by the first device varies at a predetermined rate over a predetermined time, and then concluding that the force is still being applied to the first device.
Further illustratively according to this aspect of the invention, determining the first device's output and correlating the first device's output with the force comprise detecting a relatively large signal generated by the first device, then detecting when the signal abruptly decreases below some minimum threshold, and then determining if a constant force is being applied.
According to another aspect of the invention, a monitoring system comprises a first, piezoelectric device which is subject to a force to be monitored, and a second device for determining the first device's output and for correlating the first device's output with the force.
Illustratively according to this aspect of the invention, the second device comprises a second device for providing electrical signals for exciting the first device and for determining the first device's response to the electrical signals.
Additionally illustratively according to this aspect of the invention, the second device comprises a second device for sweeping the signal frequency and determining the first device's impedance in response to the swept signal frequency.
Additionally illustratively according to this aspect of the invention, the second device comprises a switch for controlling a mode of operation of the second device for determining the first device's output and for correlating the first device's output with the force, the switch having a first state corresponding to a dynamic mode of operation when the force applied to the first device is being substantially modulated and a second state corresponding to a static mode of operation when the force applied to the first device is not being substantially modulated.
Further illustratively according to this aspect of the invention, the second device comprises a signal generator and an impedance determining circuit, switching of the switch to the second state causing a signal to be applied by the signal generator to the first device and causing the second device to determine the impedance of the first device to the applied signal.
Further illustratively according to this aspect of the invention, the second device comprises a second device for sweeping the signal frequency and determining the first device's impedance in response to the swept signal frequency.
Illustratively according to this aspect of the invention, the second device comprises a second device for determining from the impedance the force applied to the first device.
Additionally illustratively according to this aspect of the invention, the second device comprises a second device for determining from the determined force whether an object is on the first device.
Illustratively according to this aspect of the invention, the apparatus comprises a plurality of said first devices coupled together in an array so as to permit the object's location to be determined more accurately.
Additionally illustratively according to this aspect of the invention, the first device comprises a flexible piezoelectric film.
Illustratively according to this aspect of the invention, the flexible piezoelectric film comprises an array of said first devices.
Additionally illustratively according to this aspect of the invention, the first device comprises a coaxial cable including a center conductor, a shield, and between the center conductor and shield, a piezoelectric material.
Illustratively according to this aspect of the invention, the first device comprises a piezoelectric ceramic transducer.
Further illustratively according to this aspect of the invention, the first devices are formed into an array, the apparatus further comprising a third device for holding the plurality of first devices in the array on a detection surface.
Additionally illustratively according to this aspect of the invention, the piezoelectric transducer is custom fabricated to a particular size, shape, and electrical properties.
Illustratively according to this aspect of the invention, the first device is oriented at a strategic load-bearing point.
Illustratively according to this aspect of the invention, the apparatus comprises a plurality of first devices coupled together and oriented at a plurality of strategic load bearing points so as to permit an object's location to be determined more accurately.
Illustratively according to this aspect of the invention, the second device comprises a second device for detecting a relatively large signal generated by the first device, placing the second device in a second state when the relatively large signal from the first device is detected, determining if the signal generated by the first device then varies at a predetermined rate over a predetermined time, and then concluding that an object is still reposing on the first device.
Illustratively according to this aspect of the invention, the second device comprises a second device for detecting a relatively large signal generated by the first device, placing the second device in a second state when the signal abruptly decreases below some minimum threshold, immediately switching to the second mode, and determining if a constant force is being applied.